Photoelectric smoke detectors often include a mechanism to monitor an optical source which injects radiant energy into a smoke chamber of the detector. Photoelectric scattering smoke detectors often rely on a background quiescent level of optical measurement from the chamber to determine if source is operational. This is the result of light reflecting off many surfaces with the chamber and detected by a smoke sensing photo sensor. However, a very high sensitivity device, such as one having an aspirated smoke sensing chamber, may have little or no quiescent level at the respective photo sensor.